Conventional methods of slope analysis based on the concept of limit equilibrium have been widely adopted mainly due to their simplicity and applicability. Although these conventional methods are straightforward, they may not give reliable and convincing results if nonhomogeneous and anisotropic stratifications are considered. They do not take into account the type of slope history nor the initial state of stress before excavation or fill placement. In general, the slope stability analysis consists of the following two steps. The first step is to calculate a factor of safety for a specified slip surface. The next step is to find a critical surface which is associated with the minimum factor of safety. The factor of safety resulting from the limit equilibrium method is not uniquely determined. It varies with the assumption made in the analysis because of the indeterminate characteristics of the method.
The finite element method, on the other hand, has been used for the analysis of deformation and stress distribution. In contrast to the simplified techniques, the finite element approach can deal with a complex loading sequence and the growth of inelastic zone with time. This paper proposes a technique to search for the critical slip surface as well as to define and calculate the factor of safety for the slope, when the finite element method is used to model its formation. First, stresses are estimated at each Gaussian point from the finite element analysis. Then, the global stress smoothing method is applied to get a continuous stress field. Based on this stress field, the factor of safety is calculated for a specified slip surface by a stress integration scheme. An improved search strategy is proposed for a noncircular critical surface which starts with a search method for a circular critical surface. During the search process, points defining a trial slip surface can freely move in the finite element mesh subject to some kinematical constraints. This method can be applied to both the limit equilibrium method and the finite element method. Effects of the slope stress history and soil parameters on the resulting critical surface are investigated.
The stability analysis of slope with groundwater flow is conducted with FEM. The performance of a numerical technique for locating the free surface without mesh regeneration is investigated in the context of steady state pore fluid diffusion through porous and deformiable media. Due to the unknown free surface, the total unit weight of ground( saturated unit weight or dry unit weight) is not a priori determined. The 'relaxed penalty function' makes it possible to solve this difficult problem with a fixed mesh. This change of unit weight have a considerable effect on the horizontal displacement as well as the vertical displacement of a vertical cut. Also the slope stability analysis is conducted on the basis of finite element effective stress fields. The factor of safety calculated from the finite element stress fields is different from that obtained from the limit equilibrium method. This is because in the limit equilibrium method the seepage force is not clearly considered and inaccurate prediction of pore water pressure at slice bottom is used.
Finally in order to extend application of the proposed satiability theory, the stability analysis of reinforced steep slopes is conducted. For this analysis, thin interface element is used to represent the behavior of the interface between a sheet pile and a ground. This results shows that proposed theory can deal with the stability of a complex geotechnical structure.